The 25-year-old Walker pitched just two innings in his last start Saturday at Los Angeles before departing due to discomfort in his right arm. He had an MRI on Tuesday, then sought a second opinion and was headed back to Arizona on Wednesday after speaking with a doctor in New York.

Walker went 9-9 with a 3.49 ERA in 28 starts last season, helping Arizona make the playoffs as a wild card. He had no record and a 3.46 ERA in three starts this year.

Manager Torey Lovullo says Walker's injury "was significant enough that a conservative approach would not be the best bet." He says it's "terribly disappointing."

Copyright 2017 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.